The present invention relates to a method and apparatus for heating semiconductor devices and circuit boards during assembly operations. The present invention has particular applicability in wire bonding of stacked die assemblies and in flip-chip bonding.
In conventional stacked die packaging techniques, a first die is attached to a substrate, such as a metal lead frame or a laminated plastic or ceramic circuit board, as with epoxy or polyimide paste or film tape, and a second die is attached to the top surface of the first die, also with epoxy or polyimide paste or film tape. A wire bonding operation is then carried out, wherein gold or copper wires are fused to bonding pads on the top surfaces of the substrate and the dies, typically using ultrasonics and heat. To raise the substrate and dies to the proper temperature for wire bonding, typically about 190 degrees Centigrade, a heater block is commonly employed to conductively pre-heat the substrate from the bottom. In other words, although the wire bonding (i.e., soldering of the wire ends) occurs on the top surfaces of the substrate and the dies, they are heated from the bottom.
While pre-heating the substrate from the bottom using a heater block yields satisfactory results when the substrate is a thin metal lead frame having good heat conductivity, such as a copper lead frame, this conventional methodology is problematic for other types of substrates. In particular, thick laminated plastic or ceramic substrates that are slower to heat up may not be brought up to the proper temperature for wire bonding by the block heater within the available time period for pre-heating (e.g., about 4 seconds or less, depending on the speed of the wire bonder and the number of wires per die assembly). Moreover, since the dies are remote from the bottom of the substrate, their bonding areas heat up even more slowly than the substrate, and thus are less likely to be brought up to the preferred wire-bonding temperature by the block heater. Consequently, wire bond quality and manufacturing yield are adversely affected.
An analogous problem also exists in conventional flip-chip assembly methodology, wherein a semiconductor die is soldered directly onto the top surface of a substrate, typically a plastic or ceramic circuit board. Bonding pads on the substrate are coated with solder, and solder preforms called xe2x80x9csolder bumpsxe2x80x9d are attached to the die prior to assembly. Heat and pressure are used to melt the solder on both the substrate and the die to bond the die to the substrate. A typical flip-chip assembly technique employs a heater block, as discussed above, to pre-heat the substrate to partially melt the solder bumps. However, as in the case of the stacked die assembly, the heater block may not raise the temperature of the substrate top surface and the solder bumps quickly enough to ensure high-quality solder connections between the die and the substrate, thereby adversely affecting yield and reliability.
There exists a need for a methodology for ensuring adequate heating of flip-chip and wire bonding dies and substrates, thereby improving manufacturing yield and lowering costs.
An advantage of the present invention is a method and apparatus for reliably heating the bonding areas of a substrate and/or a die or dies to ensure high-quality solder or wire bonds between the substrate and the dies.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method of heating a bonding area of a top surface of a semiconductor die attached to a substrate, the method comprising heating the bonding area with infrared radiation directed at the top surface of the die.
Another aspect of the present invention is an apparatus for carrying out the above inventive method.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.